1. Field of the Invention
The present invention relates to a glass bulb for a cathode ray tube used mainly for receiving signals of TV broadcasting or the like.
2. Discussion of Background
As shown in FIGS. 1 and 2, a cathode ray tube 1 used for receiving signals for TV broadcasting or the like has a glass bulb 2 which is basically constituted by a panel glass or a panel portion 3 for displaying a picture image, a funnel portion 4 on which a deflection coil is mounted, and a neck portion 5 for housing an electron gun 17.
In FIGS. 1 and 2, reference numeral 6 designates a skirt portion in the panel portion 3, numeral 7 designates a face portion for displaying a picture image in the panel portion, numeral 8 designates an implosion-proof reinforcing band for providing strength, numeral 9 designates a blend R portion for connecting the face portion to the skirt portion, numeral 10 designates a sealing portion at which the panel portion 3 and the funnel portion 4 are sealed with solder glass or the like, numeral 12 designates a fluorescent layer for emitting fluorescence by irradiating electron beams, numeral 13 designates an aluminum film for reflecting forwardly the fluorescence at the fluorescent film, numeral 14 designates a shadow mask which specifies positions of fluorescent substance irradiated by the electron beams, numeral 15 designates a stud pin for fixing the shadow mask 14 to the inner surface of the skirt portion 6, and numeral 16 designates an inner conductive coating which prevents the shadow mask 14 from being charged to a high potential by the electron beams and which grounds electric charges to the outside.
A symbol A indicates a tube axis connecting the central axis of the neck portion 5 to the center of the panel portion 3. The fluorescent layer is formed on an inner plane of the panel glass to thereby form a screen. The screen is in substantially rectangular shape which is constituted by four lines which are in substantially parallel to a long axis and a short axis crossing at a right angle to the tube axis at the central point of the rectangular shape.
In the cathode ray tube 1 using the glass panel having a substantially box-like configuration, there are a region having a large tensile stress (a sign of "+") and a region having a compressive stress (a sign of "-") in a relatively broad area at the edge of the face portion on the short axis and the long axis, which are resulted from an asymmetric structure unlike a spherical shape, and in an outer surface of the skirt portion in the vicinity of the blend R portion, as shown in FIG. 3, because a pressure difference of 1 atmospheric pressure between the outside and the inside of the panel glass is applied thereto. In FIG. 3, a symbol .sigma..sub.R represents a component of stress along the paper surface and a symbol .sigma..sub.T represents a component of stress perpendicular to the paper surface. Numerical values described near distribution lines of stress in FIG. 3 indicate the values of stress at corresponding positions.
There is a two-dimensional distribution of stress in the front surface of the glass bulb. Generally, the maximum value of tensile vacuum stress exists in an edge portion of an image displaying surface of the face portion of the panel glass or the skirt portion of the panel glass. Accordingly, if the tensile vacuum stress produced in the glass bulb of the cathode ray tube is large and if the glass bulb does not have a sufficient strength to oppose the tensile vacuum stress, there may result a static fatigue breakage due to the atmospheric pressure and the glass bulb will not function as the cathode ray tube.
Further, in the manufacture of the cathode ray tube, the glass bulb is kept at a high temperature such as about 380.degree. C. and air inside the glass bulb is evacuated. During such heating process, a thermal stress is resulted in addition to the tensile vacuum stress. In a worse case, an intensive implosion is resulted due to an instantaneous introduction of air and the reaction thereof whereby there is a danger of causing a damage in the neighborhood. As a guarantee to prevent such breakage of the glass bulb or glass panel, an external pressure loading test has been conducted by applying a pressure to the glass bulb to which scratches are uniformly formed by using a #150 emery paper, in consideration of the depth of the scratches in the glass surface which may be produced in an assembling step of the glass bulb and the cathode ray tube, and the service life of the cathode ray tube. Then, a difference between an inner pressure and an outer pressure at the time when the glass bulb is broken is measured. The glass bulb is generally so constructed as to be durable to a pressure difference of 3 atmospheric pressure or more.
The fracture strength of the glass bulb with the scratches is not primarily determined because the tensile vacuum stress in the outer surface of the glass bulb depends on the structure of the glass bulb and has a two-dimensional distribution of stress. Generally, the fracture strength is within 18.6 MPa at the minimum value and about 24.5 MPa in average.
On the other hand, in consideration of the fatigue breakage of the glass bulb due to a vacuum stress. There is a high possibility of causing the breakage of the glass bulb in a region having the maximum tensile vacuum stress .sigma..sub.Vmax. Accordingly, the wall thickness and the shape of the glass bulb are determined so that the maximum value .sigma..sub.Vmax is in a range of from 6 to 12 MPa. Namely, the face portion is formed to have a certain extent of radius of curvature and wall thickness whereby the vacuum stress is reduced. Further, in general attempt, an edge portion of the face portion is made thicker while the face portion is not thickened as a whole whereby the vacuum stress is reduced by a wedge effect. Accordingly, the blend R portion is made thicker than the other portions.
In recent years, there is a demand of increasing the size of cathode ray tubes. In this case, when the radius of curvature of the face portion is small, there arises a problem of visibility of a picture surface. In order to eliminate the problem of the visibility, there is a proposal that the radius of curvature of the face portion be asymmetrically formed whereby the radius of curvature of the face portion can be increased by about 2 times or 3 times, and the above-mentioned range of the maximum tensile vacuum stress can be achieved without inviting a substantial increase in the thickness of the face portion. For example, when the maximum value of the outer diameter of the panel portion corresponds to that of 29-inch model, the radius of curvature of the face portion on the diagonal axis is increased to about 2400 mm while the radius of curvature on the short axis can be made small to 1400 mm. Thus, a sufficient visibility can be assured by minimizing the height difference at the peripheral portion of the face portion, and the maximum tensile vacuum stress can be reduced by reducing the radius of curvature of the face portion on the short axis.
However, when the radius of curvature of the face portion is to be further increased, for example, when the face portion is formed to have a flattened shape in terms of the 29-inch model while the above-mentioned value of the maximum tensile vacuum stress is to be maintained, the wall thickness of the face portion is increased to 18.5 mm. Therefore, Japanese Unexamined Patent Publications JP-A-7-21944 and JP-A-7-142013 propose physically strengthening is effectively conducted to a region where the tensile vacuum stress is the largest, i.e., a heat treatment is so conducted that a desired compressive stress is provided to the surface layer where the wall thickness can be reduced while the strength is maintained.
Generally, the panel glass is formed by pressing operations at a high temperature of about 1000.degree. C. Then, a physically strengthening method is conducted in such a manner that a heat treatment is applied to the glass panel so that there produces an effective temperature difference between the core and the surface of the glass at at least a temperature region which permits the rearrangement of molecules forming the glass.
In the conventional panel portion, however, the wall thickness of the blend R portion is fairly thicker than that of the face portion or the skirt portion located near the blend R portion as shown in FIG. 4. Accordingly, when the glass panel is cooled for strengthening, there is found a delay of cooling in the region adjacent to the face portion and the skirt portion which are connected to the blend R portion at which a large tensile vacuum stress is produced because the thermal capacity of the blend R portion is large and a change in the shape of the blend R portion is large. As a result, a compressive stress formed in the surface layer by the physical strengthening is smaller than that in the core of the face portion.
Accordingly, when a large stress value by strengthening is to be obtained in this region, the strengthened stress values of the core of the face portion and the seal edge portion of the skirt portion become excessive, and a tensile plane stress is newly developed in an inner surface or an outer surface of the edge portion of the face portion in order to avoid such imbalance state of the stress distribution. Further, the presence of the thick wall portion provides unstable cooling. Further, there is a problem of difficulty in controlling the strengthened stress value in this region.